And it came to pass in those days, that the Philistines gathered their armies together for warfare, to fight with Israel. And Achish said unto David, Know thou assuredly, that thou shalt go out with me to battle, thou and thy men. The Philistines gathered their armies together - Sir Isaac Newton conjectures that the Philistines had got a great increase to their armies by vast numbers of men which Amasis had driven out of Egypt. This, with Samuel's death, and David's disgrace, were no inconsiderable motives to a new war, front which the Philistines had now every thing to hope.
Thou shalt go out with me to battle - This he said, being deceived by what David had told him.